1. Field of the Invention
The present invention is related to the field of geophysical exploration and more specifically to a method of receiving seismic signals on a drill string in a wellbore to acquire seismic data while drilling.
2. Description of the Related Art
In drilling a borehole to recover oil from the earth, it is often helpful to turn or steer the downhole drill bit toward or away from subterranean targets. To facilitate geophysical steering, drillers need to know drill bit location on the seismic section. The location of targets ahead of the bit is also required, as well as some warning or indication of drilling hazards such as over-pressured formations or thin, shallow gas intervals. Surface seismic surveys generally include this information, but resolution and depth location is poor because surface seismic surveys are time based (rather than depth based) and the low frequencies generally available are not conducive for high resolution. For example, to determine the depth of a reflection, a speed of sound for the earth formation must be known. Consequently, these systems require depth calibration to accurately determine locations of target horizons or drilling hazards. Traditionally, this calibration has been provided by either offset well sonic data or wireline checkshot data in the current well. Offset data is often inadequate however due to horizontal variations in stratigraphy between wells.
During surface seismic surveys, a plurality of seismic sources and seismic receivers are placed on the surface of the earth. The seismic sources are triggered in a predetermined sequence, resulting in the generation of seismic waves. These seismic waves travel downward through the earth until reflected off some underground object or change in rock formation. The reflected seismic waves then travel upward and are detected at the seismic receivers on the surface. One or more clocks at the surface measure the time from generation of the seismic waves at each source to the reception of the seismic waves at each receiver. This gives an indication of the depth of the detected object underground. However, the exact speed of sound for these seismic waves is unknown, and thus, the exact depth of the detected object is also unknown. To more closely measure the exact speed of sound, a “wireline checkshot” acquired during drilling operations may be used to calibrate depth measurements by measuring the transit times between seismic sources and seismic receivers. During a “wireline checkshot,” a receiver on a “wireline” is lowered a known distance into an already-drilled borehole. A surface seismic source is then triggered and the time is measured for the seismic wave to travel to the wireline receiver. Because the depth of the wireline receiver is known, an average interval velocity indicating the average speed of the seismic wave can be determined with some degree of accuracy. Wireline checkshots, however, require interruption of the drilling operations by removing the drilling string out of the hole, commonly known as tripping, and so wireline checkshot surveys are often prohibitively expensive.
With Vertical Seismic Profiling (VSP), acquisition is conducted between one or more seismic sources on or near the surface of the earth, and receivers in the earth, for example in a wellbore. Measurements are acquired of the response of a seismic sensor (like a geophone) at various depths in a borehole to sources on the surface. Sometimes the surface sources are moved about the area as well. Where the source is an appreciable distance from the well head, the result is termed an offset VSP; this provides a way to seismically “look” to the side of the borehole in the vicinity around the adjacent earth formation. For an “azimuthal VSP,” where the velocity field is sampled for azimuthal velocity changes relative to different lateral directions, sources are offset on the surface in different (i.e., various) directions from the borehole. In a walk-away VSP a surface source moves while the geophone in the borehole remains stationary; this provides another way to look to the side of the borehole. In a reverse VSP the source is in the borehole and geophones are on the surface. In a reverse VSP the noise generated by the drill bit during drilling operations may be used as a seismic source. VSPs are also acquired in directional and horizontal wells. A walk-above VSP is made with the sonde in a deviated hole and the source moved so as to be vertically above it. Many issues with VSP and other near borehole acoustic surveys concern the multiplicity of various types of background noise present in the environment that may substantially interfere with signal reception.
US patent application 20030144797 to Leaney discusses a method of performing the time picking step in a VSP (vertical seismic profile) survey. The time picking step is carried out on a combined three-component amplitude of the received seismic energy, which contains the amplitude of all the seismic energy received at the receiver. The amplitude of the direct pulse in the combined trace will not decrease to zero as the offset of the source is changed, as can be the case for the single-component amplitude of the direct pulse. In a particularly preferred embodiment of the invention, the combined three-component amplitude is calculated by summing the Hilbert instantaneous amplitudes of the x-, y- and components of the seismic data using the equation (I). The Leaney application also provides two new time picks. One time pick involves finding the maximum positive gradient of A(t). The other time pick entails extrapolating to A(t)=0 from the time at which A(t) has its greatest positive gradient, using the maximum positive gradient for the extrapolation.
US patent application 20040122593 to Haldorsen discloses a method of processing multi-component seismic measurement-while-drilling data that includes rotating the data to align energy in the data along a common axis and then combining the data. Another embodiment of the inventive method involves a technique for estimating the orientations and/or change in orientation of a bottom hole assembly associated with two or more series of multi-component seismic MWD data acquired at different times at the same source and receiver locations. Embodiments of the inventive method may be used to improve the signal to noise ratio of the data, to reduce downhole storage and transmission requirements, and to improve direct arrival time picks made using the data.
There is a need to acquire seismic data, both checkshot transit time and VSP data during drilling operations without the expense of interrupting and halting the drilling operations while enhancing the desired signal energy and suppressing the environmental noise energy. There is a need for a method and system of receiving accurate high resolution checkshot and VSP seismic data in the vicinity of the drilling string generally and the drill bit particularly without the need to halt drilling operations.
The methods of the present invention overcome the foregoing disadvantages of the prior art by providing a technique for deploying a wellbore seismic receiver in a drill string without the use of a wireline and acquiring seismic data as the drill string operations are conducted within the wellbore.